One of the biggest problems caused from vehicles using fossil fuel, such as gasoline and diesel oil, is creation of air pollution. A technology of using a secondary battery, which can be charged and discharged, as a power source for vehicles has attracted considerable attention as one method of solving the above-mentioned problem. As a result, electric vehicles (EV), which are operated using only a battery, and hybrid electric vehicles (HEV), which jointly use a battery and a conventional engine, have been developed. Some of the electric vehicles and the hybrid electric vehicles are now being commercially used. A nickel-metal hydride (Ni-MH) secondary battery has been mainly used as the power source for the electric vehicles (EV) and the hybrid electric vehicles (HEV). In recent years, however, the use of a lithium-ion secondary battery has been attempted.
High output and large capacity are needed for such a secondary battery to be used as the power source for the electric vehicles (EV) and the hybrid electric vehicles (HEV). For this reason, a plurality of small-sized secondary batteries (unit cells) are connected in series or in parallel with each other so as to construct a battery module, and a plurality of battery modules are connected in parallel or in series with each other so as to construct a battery pack.
In such a high-output, large-capacity secondary battery, however, a large amount of heat is generated from the unit cells during the charge and the discharge of the unit cells. When the heat generated from the unit cells during the charge and the discharge of the unit cells is not effectively removed, heat is accumulated in the unit cells with the result that the unit cells are degraded. Consequently, it is necessary to provide a cooling system for vehicle battery packs, which are high-output, large-capacity secondary batteries.
Generally, a cooling system for vehicle battery packs is constructed in an air-cooling structure using air as a coolant. In the air-cooling structure, air outside or inside the vehicle is introduced to cool the battery pack, and is then discharged out of the vehicle. Consequently, various technologies for improving the efficiency of the cooling system have been developed.
For example, Japanese Unexamined Patent Publication No. 1996-148189 discloses a temperature control system in which heat transfer plates are attached to battery cells, respectively, heat dissipating fins are mounted at the outside of each heat transfer plate, and a cooling fan is operated depending upon the temperature of batteries, whereby the batteries are cooled. In addition, Korean Unexamined Patent Publication No. 2005-0018518 discloses a cooling system in which heat absorbing members are attached to batteries, respectively, and the batteries are cooled using air introduced by a cooling fan. These technologies improve the cooling efficiency of the batteries using the heat dissipating fins and the heat absorbing members in their own ways. However, the mounting of the heat dissipating fins at the respective battery cells or the attachment of the heat absorbing members to the respective battery cells complicates the overall construction of the corresponding system. As a result, the assembly process of the corresponding system is very complicated. In addition, the cooling efficiency is not obtained to a desired degree although such attempts are made as described above.
Meanwhile, the vehicles, such as the electric vehicles (EV) and the hybrid electric vehicles (HEV), may be operated under the tough conditions. For example, when a vehicle is at low temperature in the winter season, it is necessary to stop the operation of a cooling system such that a battery pack can be operated at the optimum operating temperature. Alternatively, it may be necessary to increase the temperature of air introduced into the battery pack such that the operating temperature of the battery pack can be controlled to the optimum temperature level. In this case, it is necessary to provide an additional device. In addition, when the unit cells of the battery pack already have low temperature, battery components may be damaged. Furthermore, the degradation of the battery components may be accelerated due to the abrupt increase of the temperature of the air introduced into the battery pack.